U.S. patent number 7,358,507 [Application Number 10/538,115] was granted by the patent office on 2008-04-15 for liquid removal in a method and device for irradiating spots on a layer.
This patent grant is currently assigned to Koninklijke Philips Electronics N.V.. Invention is credited to Helmar Van Santen.
United States Patent |
7,358,507 |
Van Santen |
April 15, 2008 |
Liquid removal in a method and device for irradiating spots on a
layer
Abstract
For irradiating a layer (3) a radiation beam (7) is directed and
focused to a spot (11) on the layer (3), relative movement of the
layer (3) relative to the optical element (59) is caused so that,
successively, different portions of the layer (3) are irradiated
and an interspace (53) between a surface of the optical element
(59) nearest to the layer (3) is maintained. Furthermore, at least
a portion of the interspace (53) through which the radiation
irradiates the spot (11) on the layer (3) is maintained filled with
a liquid (91). By directing a gas flow (71-73) to a surface zone
(74) of the layer (3), liquid (91) is reliably prevented from
passing that surface zone (74), without causing damage to the layer
(3). The liquid (91) is drawn away from the layer (3) in the
vicinity of the surface zone (74).
Inventors: |
Van Santen; Helmar (Eindhoven,
NL) |
Assignee: |
Koninklijke Philips Electronics
N.V. (Eindhoven, NL)
|
Family
ID: |
32524023 |
Appl.
No.: |
10/538,115 |
Filed: |
November 14, 2003 |
PCT
Filed: |
November 14, 2003 |
PCT No.: |
PCT/IB03/05200 |
371(c)(1),(2),(4) Date: |
June 08, 2005 |
PCT
Pub. No.: |
WO2004/055803 |
PCT
Pub. Date: |
July 01, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060261288 A1 |
Nov 23, 2006 |
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Foreign Application Priority Data
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Dec 13, 2002 [EP] |
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02080274 |
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Current U.S.
Class: |
250/492.1;
359/581; 359/619; 250/235; 250/234; 430/311; 430/322; 369/44.14;
250/216; G9B/7.195 |
Current CPC
Class: |
G03F
7/70341 (20130101); G11B 7/1374 (20130101); G11B
7/122 (20130101); G11B 7/261 (20130101) |
Current International
Class: |
G21G
5/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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09153233 |
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Jun 1997 |
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JP |
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10255319 |
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Sep 1998 |
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JP |
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WO9949504 |
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Sep 1999 |
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WO |
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WO0213194 |
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Feb 2002 |
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WO |
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Primary Examiner: Berman; Jack I.
Assistant Examiner: Hashmi; Zia R.
Attorney, Agent or Firm: Belk; Michael E.
Claims
The invention claimed is:
1. A method of irradiating a layer (3) including: directing and
focusing a radiation beam (7) to a spot (11) on said layer (3) by
means of at least one optical element (59); causing relative
movement of the layer (3) relative to said at least one optical
element (59) so that, successively, different portions of the layer
(3) are irradiated and an interspace (53) between a surface of said
at least one optical element (59) nearest to said layer (3) is
maintained; and maintaining at least a portion of said interspace
(53) through which said radiation irradiates said spot (11) on said
layer (3) filled with a liquid (91) supplied via a supply conduit;
characterized by directing gas (71-73) to said layer (3); and
removing supplied liquid (91) from said layer (3) in the vicinity
of a flow of said gas (71-73).
2. A method according to claim 1, wherein said gas (71-73) is
supplied at a pressure sufficiently high to cause a net gas flow
(71-73) in a direction along said layer (3) opposite to the
direction (30) of said movement of said layer (3).
3. A method according to claim 1 or 2, wherein the flow of said gas
(71-73) is entered into an interspace between said layer (3) and a
boundary surface (83) having a width of at least 2 .mu.m and
preferably at least 5 .mu.m and at most 100 .mu.m and preferably 30
.mu.m.
4. A method according to claim 3, wherein the liquid (91) forms a
film on said layer (3) having a thickness, and wherein an
interspace (86) between said layer (3) and a surface (87) facing
said layer (3) upstream of an area where the liquid is discharged
is larger than the thickness of said film.
5. A method according to any one of the preceding claims, wherein
liquid (91) and gas are drawn away from said layer (3) at a higher
flow rate than the sum of the flow rates of said gas flow (71-73)
and the supply of said liquid (91).
6. A method according to any one of the preceding claims, wherein
said gas (71-73) is air.
7. A device for directing radiation to a layer (3) including: at
least one optical element (59) for focusing a beam (7) of radiation
originating from a radiation source (33) to a spot (11) on said
layer (3); a displacement structure for causing relative movement
of the layer (3) relative to said at least one optical element (59)
so that, successively, different portions of the layer (3) are
irradiated and an interspace (53) between said layer (3) and a
surface of said at least one optical element (59) nearest to said
spot (11) is maintained; and an outflow opening for supplying
liquid (91) to at least a portion of said interspace (53) through
which, in operation, said radiation irradiates said spot (11) on
said layer (3); characterized by a gas outflow opening (70) for
directing a gas flow (71-73) to said layer (3); and a discharge
channel (76) having an inlet (77) in the vicinity of said gas
outflow opening (70) for drawing away liquid (91) from the layer
(3).
8. A device according to claim 7, wherein said gas outflow opening
(70) for directing said gas flow (71-73) is a slit.
9. A device according to claim 7 or 8, wherein said discharge
channel (76) communicates with a vacuum source (81).
10. A device according to any one of the claims 7-9, wherein said
gas outflow opening (70) and said inlet (77) of said
discharge-channel (76) extend about said interspace (53).
Description
The invention relates to a method of irradiating a layer according
to the introductory portion of claim 1 and to a device for
irradiating a layer according to the introductory portion of claim
7.
In several embodiments of liquid immersion in dynamic systems,
liquid immersion is maintained through continuously supplying
liquid. A liquid film is maintained between the lens and the object
by constantly supplying liquid through a first supply conduit, e.g.
a hole, just upstream of the immersion lens at a sufficiently high
pressure to avoid gas inclusion. The moving surface pulls the
liquid to the image field, herewith ensuring the imaging field to
be immersed. Even though with a careful design of the immersion
system liquid flow can be kept low, still continuously liquid is
supplied. For stable continuous operation, it is advisable to
remove the liquid.
In WO-A-02/13194 a method and a system of the initially identified
type are proposed to remove the liquid, related to liquid immersion
mastering of optical discs. According to this publication, first a
master mold is manufactured, and then, by means of the master mold
or by means of a daughter mold manufactured by means of the master
mold, an optically scannable information carrier is manufactured by
means of a replica process. For manufacturing the master mold, a
modulated radiation beam which is directed and focused to a
scanning spot on a photosensitive layer carried by a substrate by
means of an optical lens system and the substrate and the lens
system are moved relatively to each other. An interspace between
the photosensitive layer and a nearest surface of a lens system
facing the photosensitive layer is maintained filled up with a
liquid.
For moving the substrate relative to the lens system a table
carrying the substrate is rotated about an axis of rotation. By
means of a displacement device, the lens system can be displaced
with a radial directional component with respect to the axis of
rotation of the table. A liquid supply means supplies the liquid
into the interspace between the photosensitive layer and a nearest
optical surface of the lens system.
Another method and device for directing a radiation beam to a spot
on a photosensitive layer, in which an interspace between the lens
and the layer is maintained filled with liquid is disclosed in
JP-A-10255319.
It is also known to maintain a gap between a lens and a surface to
be irradiated filled with a liquid in optical imaging methods and
apparatus, such as optical projection lithography, in which the
spot formed by the radiation projected onto the surface is an image
or a part of an image. Such a method and apparatus are described in
international patent application WO99/49504.
The liquid immersion in dynamic systems, such as wafer steppers in
optical lithography and mastering machines in optical disc
manufacturing may be maintained by a continuous supply of liquid.
Preferably, a liquid film is maintained between the lens and the
object by constantly supplying liquid through a hole just upstream
of the immersion lens at a sufficiently high pressure to avoid gas
inclusion. The moving surface entrains the liquid to the image
field, ensuring that interspace between the imaging field (the
spot) and the optical element closest thereto is maintained
immersed. Even though with a careful design of the immersion system
liquid flow can be kept low, still continuously liquid is supplied.
For stable continuous operation, it is advisable to remove the
liquid. However, a key problem is to break up the adhesive forces
between the liquid and the substrate surface without damaging the
surface of the substrate. The substrate layer is often very
delicate, for instance a soft resist layer.
Imaging systems such as wafer steppers and optical disc mastering
equipment usually are very sensitive to mechanical disturbances.
Varying forces exerted by the liquid film easily cause undesirable
vibrations, which disturb the precision with which the image is
projected onto the surface.
It is an object of this invention to reliably remove liquid of a
thin liquid film from a substrate without damaging the surface and
minimizing mechanical disturbances, in an imaging system for
irradiating a surface, such as for example wafer steppers and
optical disc mastering equipment.
According to the invention, this object is achieved by providing a
method according to claim 1. Also according to the invention, a
device according to claim 7 is provided for carrying out a method
according to claim 1.
Using a gas flow directed towards the layer, adhesion between the
liquid and the layer can be broken up very effectively, without
damaging the layer and the liquid can be removed easily from the
vicinity of the gas flow where the adhesion is broken up. The gas
flow pushes the liquid from the disc. An additional advantage of
this system is that most non-sticking particles will be removed
too.
Also according to the invention, the gas may be pumped through a
second supply conduit at a pressure sufficiently high to cause a
net gas flow in the direction opposite to the direction of relative
movement of the layer in order to remove the liquid from the layer
through a drainage conduit.
Particular embodiments of the invention are set forth in the
dependent claims. Other objects, features and effects as well as
details of this invention appear from the detailed description of a
preferred form of the invention.
FIG. 1 is a schematic side view of an example of a device for
directing radiation to a spot on a layer;
FIG. 2 is a schematic, cross-sectional view of a distal end portion
of an example of an optical system for a device as shown in FIG. 1,
of a layer to which the radiation is directed and of a liquid flow
maintained in operation;
FIG. 3 is a schematic, bottom view along the line III-III in FIG.
2;
FIG. 4 is an enlarged representation of a portion of FIG. 2
including the zone to which a gas flow is directed;
FIG. 5 is a schematic top plan view representation of a wafer
stepper/scanner for optical lithography.
In the manufacture of an optically scannable information carrier,
such as a CD or a DVD, a disc-shaped substrate 3 of glass (see FIG.
1) carrying a thin photosensitive layer on one of its two sides is
irradiated by means of a modulated radiation beam 7, for instance a
DUV laser beam with a wavelength of approximately 260 nm. According
to the example shown in FIG. 1, the photosensitive layer 5 is
irradiated using a device 25, which is described hereinafter with
reference to FIGS. 1-4. The radiation beam 7 is focused to a
scanning spot 11 on the photosensitive layer 5 by an optical system
in the form of a lens system 9, including a plurality of optical
elements. The lens system 9 includes a most distal lens 59, which
is the one of the optical elements of the lens system 9 that is
located nearest to the layer 5 when in operation. An interspace 53
is maintained between the layer 5 that is irradiated and the one 59
of the optical elements of the lens system 9 that is located
nearest to the layer 5. The optical elements may also include other
items than lenses, such as filters, shields, diffraction gratings
or mirrors.
The layer 5 and the lens system 9 are displaced with respect to
each other, so that the modulated radiation beam 7 on the
photosensitive layer 5 successively irradiates a series of spots on
the layer 5. The irradiated photosensitive layer 5 is subsequently
developed by means of a developing liquid, which dissolves the
irradiated portions and leaves the non-irradiated portions of the
layer on the substrate 3. It is also possible to provide that the
irradiated portions are left while the non-irradiated portions are
dissolved. In both cases, a series of pits or bumps, which
corresponds to the desired series of pit-shaped information
elements on the information carrier, are formed in the
photosensitive layer 5. The photosensitive layer 5 is subsequently
covered with a comparatively thin layer of for instance nickel by
means of a sputtering process. Subsequently, this thin layer is
covered with a comparatively thick nickel layer in an electro
deposition process. In the nickel layer, which is eventually
removed from the substrate 3, the pattern of pits formed in the
photosensitive layer 5 leaves a corresponding pattern that is a
negative of the pattern to be formed in the information carrier to
be manufactured, i.e. the master mold comprises a series of raised
portions, which correspond to the series of pit-shaped elements
formed in the photosensitive layer 5 and to the desired series of
pit-shaped information elements on the information carrier. The
master mold is thus rendered suitable for use as a mold in an
injection-molding machine for injection molding the desired
information carriers. Generally, however, a copy of the master mold
is used as the mold for injection molding instead of the master
mold, which copy of the master mold is commonly referred to as
daughter mold, which is manufactured by means of the master mold
using a customary replica process which is known per se.
The substrate 3 with the photosensitive layer 5 is carried by a
table 27 that is rotatable about an axis of rotation 29, which
extends perpendicularly to the table 27 and the substrate 3. The
table can be driven by means of a first electromotor 31. The device
25 further comprises a radiation source 33, which, in the example
shown, is a laser source, which is secured in a fixed position to a
frame 35 of the device 25. It is observed that, as an alternative,
the radiation may also be obtained from outside the device. Control
over the radiation directed to the layer 5 can be achieved in many
ways, for instance by controlling the radiation source 33 and/or by
controlling a shutter or radiation diverter (not shown) between the
radiation source 33 and the layer 5.
The optical lens system 9 is secured onto a first traveler 37,
which can be displaced radially (parallel to the X-direction in the
drawings) relative to the axis of rotation 29, by means of a first
displacement structure 39. For this purpose, the first displacement
structure 39 includes a second electromotor 41 by means of which
the first traveler 37 can be displaced over a straight guide 43,
which extends parallel to the X-direction and is fixed relative to
the frame 35.
A mirror 45 in line with an optical axis 49 of the lens system 9 is
also secured to the first traveler 37. In operation, the radiation
beam 7 generated by the radiation source 33 follows a radiation
beam path 47 extending parallel to the X-direction, and the
radiation beam 7 is deflected by the mirror 45 in a direction
parallel to the optical axis 49 of the lens system 9. The lens
system 9 can be displaced in the direction of its optical axis 49
by means of a focus actuator 51, over comparatively small distances
with respect to the first traveler 3, so that the radiation beam 7
can be focused on the photosensitive layer 5. The table 27 with the
substrate 5 is rotated about the axis of rotation 29 at a
comparatively high speed by means of the first motor 31, and the
lens system 9 is displaced parallel to the X-direction by means of
the second motor 41 at a comparatively low speed, so that the
scanning spots 11 where the radiation beam 7 hits the layer form a
spiral-shaped trail over the photosensitive layer 5 of irradiated
and non-irradiated elements.
The device 25 can suitably be used to manufacture master molds
having a comparatively high information density, i.e. by means of
the device 25, a comparatively large number of irradiated elements
can be provided per unit area of the photosensitive layer 5. The
attainable information density increases as the spot 11 is smaller.
The size of the spot 11 is determined by the wavelength of the
radiation beam 7 and by the numerical aperture of the lens system
9, the numerical aperture depending upon the optical refractive
index of the medium present between the lens system 9 and the
photosensitive layer 5. A smaller size of the spot 11 is attainable
as the refractive index of the medium present between the lens
system 9 and the photosensitive layer 5 is larger. Liquids
typically have a much larger optical refractive index than air and
therefore the portion of the interspace 53 between the lens system
9 and the photosensitive layer 5 through which the beam 7 extends
is maintained filled with a liquid--according to this example
water. In the present example, water is also particularly suitable
because it is transparent to the DUV radiation beam 7 used and it
does not attack the photosensitive layer 5. However, throughout the
present detailed description, where water is mentioned it may also
be replaced by any other suitable liquid.
FIGS. 2 and 3 show, in more detail, the lens system 9, the
substrate 3 with the photosensitive layer 5, and the interspace 53
between the photosensitive layer 5 and the lens system 9. The lens
59 nearest to the layer 5 has an optical surface 63 facing the
substrate 3 and nearest to the substrate 3. The lenses 55, 59 are
suspended in a housing 61, which includes a flat surface 65, which
faces the layer 5 and which substantially extends in a plane
perpendicular to the optical axis of the lens 59 nearest to the
layer 5.
In operation, the portion of the interspace 53 through which the
radiation 7 irradiates the spot 11 on the layer 5 is maintained
filled with water 91. The water 91 is, at least to some extent,
protected against being entrained from the interspace 53 in a
recess 92 in the lens system 9 facing the layer 3.
The optimum working distance between the layer 5 and the wall 65,
i.e. the portion of the lens assembly nearest to the layer 5, is
determined by two factors. On the one hand, the distance should be
large enough to retain sufficient tolerance on the distance between
the substrate 3 and arrangement of the lenses 55, 59 and the
housing 61. On the other hand, this distance should not be too
large, because this would require a too large water flow to
maintain the immersed condition of the portion of the interspace 53
through which the radiation passes to the spot 11. A presently
preferred range for the smallest thickness of the interspace 53 is
3-1500 .mu.m and more preferably 3-500 .mu.m. Larger values for the
smallest thickness of the interspace can be particularly
advantageous if the liquid has a larger viscosity than water. Also
the overall width of the outflow opening affects the upper end of
the preferred range for the smallest thickness of the interspace,
the smallest thickness of the interspace being preferably smaller
than (100+ 1/20*W).mu.m in which W is the overall width of the
outflow opening measured in a plane parallel to the layer 5. The
smallest thickness of the interspace may be larger than
approximately 10 .mu.m, for instance larger than 15 .mu.m, 30 .mu.m
or even 100 .mu.m, to increase the insensitivity to tolerances.
The outflow opening 90, is at least to a large extent, centered
relative to the portion of the interspace 53 through which
radiation passes to the spot 11. Accordingly, the direction of
movement of the layer 5 and the lens arrangement 9 relative to each
other in the area of the spot 11 can be varied substantially
without disrupting complete immersion of the portion of the
interspace 53 through which the spot 11 is irradiated.
The more the direction of movement of the layer 5 and the lens
system 9 parallel to the layer 5 in the area of the spot 11 can be
changed without disrupting the immersion of the portion 94 (see
FIG. 3) of the interspace 53 through which the radiation actually
passes, the more the device is suitable for applications in which
the spot 11 needs to move over the surface of the layer in widely
varying directions, such as in imaging processes in which the spot
is a two-dimensional image projected to the layer 5. In such
applications, the advantage of a comparatively large refractive
index between the lens system and the medium between the lens
system and the irradiated surface is that the image can be
projected with a higher resolution, which in turn allows further
miniaturization and/or an improved reliability.
An example of such applications is optical projection lithography
for the processing of wafers for the manufacture of semiconductor
devices. An apparatus and a method for this purpose are
schematically illustrated in FIG. 5. Wafer steppers and wafer
scanners are commercially available. Accordingly, such methods and
apparatus are not described in great detail, but primarily to
provide an understanding of water immersion as proposed in the
present application in the context of such optical imaging
applications.
The projection lithography apparatus according to FIG. 5 includes a
wafer support 12 and a projector 13 having a lens assembly 14 above
the wafer support 12. In FIG. 5, the wafer support 12 carries a
wafer 15 on which a plurality of areas 16 are intended to be
irradiated by a beam projecting an image or partial image of a mask
or reticle 17 in a scanner 18 operatively connected to the
projector 13. The support table is moveable in X and Y direction
along spindles 19, 20 driven by spindle drives 21, 22. The spindle
drives 21, 22 and the scanner 18 are connected to a control unit
23.
Usually one of the following two principles of operation is applied
in optical lithography. In the so-called wafer stepper mode, the
projector projects a complete image of the reticle onto one of the
areas 16 on the wafer 15. When the required exposure time has been
reached, the light beam is switched off or obscured and the wafer
15 is moved by the spindle drives 21, 22 until a next area 16 of
the wafer is in the required position in front of the lens assembly
14. Dependent on the relative positions of the exposed area and the
next area to be exposed, this may involve relatively quick movement
of the lens assembly 14 along the surface of the wafer in widely
varying directions. The size of the irradiated spot on the surface
of the wafer in which the image of the reticle is projected is
typically about 20.times.20 mm, but larger and smaller spots are
conceivable.
In particular when it is desired to manufacture larger
semiconductor units, it is advantageous to project the image in the
other mode, usually referred to as the wafer scanner mode. In that
mode, only a slit-shaped portion of the reticle is projected as a
slit shaped spot having a length that is several (for instance four
or more) times larger than its width in an area 16 on the surface
of the wafer 15. A typical size for the spot is for instance
30.times.5 mm). Then, the reticle 17 to be scanned is moved along
the scanning window while the wafer support 12 is synchronously
moved relative to the lens assembly 14 under control of the control
unit 23 with a velocity adapted so that only the projection spot,
but not the scanned partial image portions of the reticle 17 that
are projected on the wafer move relative to the wafer 15. Thus, the
image of the reticle 17 is transferred to an area 16 of the wafer
as successive portions "unroll" as the spot progresses over the
wafer. The movement of the wafer 15 relative to the lens assembly
14 while a running window portion of the reticle is projected onto
the wafer 15 is usually carried out relatively slowly and usually
each time in the same direction. After the complete image of a
reticle 17 has been projected onto the wafer 15, the wafer 15 is
generally moved much more quickly relative-to the lens assembly 14
to bring a next area of the wafer 15, where a next image of the
reticle 17 is to be projected, in front of the lens assembly 14.
This movement is carried out in widely varying directions dependent
on the relative positions of the exposed area 16 of the wafer 15
and the next area 16 of the wafer 15 to be exposed. To be able to
recommence irradiating the surface of the wafer 15 after the
displacement of the wafer 15 relative to the lens 14 (i.e. also the
lens or the lens and the wafer may be moved), it is advantageous if
the water volume in the interspace between the lens 14 and the
surface of the wafer 15 through which the radiation passes is
immediately filled with water after completion of that movement, so
that the space is reliably immersed before radiation is
recommenced.
Also for optical lithography, water can be used, for instance if
the radiation is light of a wavelength of 193 mm. However, in some
circumstances other liquids may be more suitable.
For supplying water 91 to the interspace 53 between the lens 59 and
the layer 5, a water supply conduit 67 extends through the housing
61 and leads to an outflow opening 90. According to the present
example, the outflow opening 90 has the form of a slit in a surface
54, which slit 90 is open towards the layer 5, for distributing
supplied water 91 longitudinally along the slit 90 and dispensing
distributed water towards the layer 5. In operation, the water 91
is distributed via the slit 90 longitudinally along that slit and
the water 91 is dispensed from the slit 90 towards the layer 5.
This results in a relatively wide water trace 95 and full immersion
of the portion 94 of the interspace 53 through which the radiation
beam 7 passes, even if the direction of movement of the lens system
9 and the layer 5 relative to each other parallel to the plane of
the layer 5 is changed substantially.
The slit 90 can have various forms. In the embodiment shown in
FIGS. 2 and 3, the slit is formed such that the outflow opening 90
is located outside the radiation beam 7 and extends around the
portion 94 of the interspace 53 through which the radiation 7
irradiates the spot 11. The cross 96 indicates the center, seen in
a direction parallel to the optical axis of the lens system 9, of
the total cross-sectional passage area of the outflow opening
90.
The water 91 is preferably supplied at a pressure drop between the
slit 90 and the environment that is just sufficient to keep portion
of the interspace 53 through which the radiation passes reliably
immersed. Thus, the amount of water fed to the surface is kept to a
minimum.
Furthermore, when the water 91 is dispensed via a slit-shaped
outflow opening 90, the smallest thickness of the interspace 153
(in this example the distance between the layer 5 and the surface
54 of the wall portion 65) may be larger, without causing an undue
risk of disrupting the immersion of the portion 94 of the
interspace through which the radiation passes.
The flow rate at which the water 91 is supplied is preferably such
that it can be reliably ensured that a laminar flow with an
essentially linear velocity profile is present in the interspace
53. Such a flow exerts a substantially constant force on the wall
65 in which the canal 90 is provided and on the side 63 of the lens
59 nearest to the layer 5. As a result, the water present in the
interspace 53 exerts substantially no variable water forces on the
lens system 9. Such varying water forces may lead to undesirable
vibrations of the lens system 9 and hence to focusing errors and
positioning errors of the radiation beam 7 on the photosensitive
layer 5. The flow is preferably free of air inclusions, so that the
radiation beam 7 is not disturbed.
The water that has been supplied to the layer 3 also has to be
removed. The key problem herein is to break up the adhesive forces
between the liquid and the surface. In particular when the surface
moves fast, such as during the manufacture of masters for
manufacturing optical discs, or during movement of a wafer between
positions where reticles are to be created, this break-up requires
large forces. The surface of the layers, however, usually is very
delicate, often a soft resist layer. Moreover, imaging systems such
as wafer steppers and optical disc mastering equipment usually are
very sensitive to mechanical disturbances. Two phase flow of liquid
and gas, which is difficult to avoid in dynamic liquid immersion
systems usually is accompanied by mechanical disturbances.
A presently most preferred embodiment of an arrangement and method
according to the invention for removing the thin liquid film
without damaging the surface while minimizing mechanical
disturbances is illustrated in FIGS. 2 and 3.
An air outflow opening 70 is provided for directing an airflow
(arrows 71, 72, 73) to a zone 74 of said layer 3 preventing water
91 from passing that zone 74. According to the present example, the
air outflow opening communicates with an air supply source 75 as is
schematically shown in FIG. 2. The air supply source 75 may for
instance include a ventilator or an air pump and may include a
reservoir in which air is stored under pressure. Instead of air,
the reservoir may also contain a specific gas or mixture of gases.
Furthermore, an arrangement for detecting the flow rate of the air,
such as a valve or a heated wire and a control valve for
controlling the flow rate in accordance with differences between
the measured flow rate and the desired flow rate may be
provided.
The airflow is preferably supplied at a pressure sufficiently high
to cause a net airflow in a direction along the layer 3 opposite to
the direction 30 of the movement of the layer 3 relative to the
optical system 9. It is thus ensured that water is pushed off the
layer 3 and does not reach the portion of the zone 74 where the
airflow 71 is bent in different directions 72, 73 and where water
might also be urged away from the immersed interspace 53 instead of
being restrained to stay in an area closely about the immersed
interspace 53.
Furthermore, a discharge channel 76 having an inlet 77 in the
vicinity of the air outflow opening 70, and therefore of zone 74,
is provided for drawing away (arrows 78, 79, 80) water 91 from the
layer 3.
In operation, the airflow 71-73 that is directed to the zone 74 of
the layer 3 prevents water 91 from passing that zone 74 by breaking
up the adhesion between the water 91 and the layer 3 and forming a
barrier in which the pressure is high enough to prevent the water
91 from passing through the zone 74. At the same time, in the
vicinity of the zone 74, the water 91 that is prevented from
passing the zone 74 is drawn away from the layer 3 via the
discharge channel 76. Thus, the initial separation of water 91 from
the layer 3 by blowing the water off the layer 3 in the area of the
zone 74 is followed by discharge of that water via the discharge
channel 76. Since the water 91 is separated from the layer 3 by
airflow, the risk of causing damage to the layer 3 is virtually
non-existent.
The air outflow opening 70 for directing the airflow 71-73 is a
slit. This allows to direct the airflow towards the layer along an
elongate zone which is particularly effective for preventing water
91 from displacing away from the optical system 9 beyond the zone
74, in particular if at least a portion of the slit 70 extends
transversely to the direction of movement 30 of the layer 3
relative to the optical system 9. The movement of the layer 3 in
the direction 30 then entrains the water 91 applied to the layer 3,
until it reaches the zone 74 where the airflow 71-73 is directed
towards the layer 3. There the airflow 71-73 causes the water 91 to
be separated from the layer 3 and the separated water is drawn away
from the layer 3.
As is best seen in FIG. 3, the air outflow opening 70 and the inlet
77 of the discharge channel 76 extend about the interspace 53
between the layer 3 and the closest one 59 of the lenses of the
optical system 9 that is maintained filled with water. This allows
to reliably prevent applied water 91 from escaping from the area
under the optical system 9 independently of the direction (parallel
to the layer 3) in which the layer 3 is moved, without having to
resort to more complicated and/or less rigid alternative solutions,
such as directing the airflow selectively from one or more of a
plurality of outflow openings or rotating an outflow head about the
optical axis of the lens system 9 to keep the outflow opening in a
position downstream of the immersed interspace. The circular shape
of the air outflow slit 70 and the inlet slit 77 provides for an
even distribution of the air outflow and of the suction completing
the removal of water from the layer 3.
As is schematically indicated in FIG. 2, the discharge channel 76
connects to vacuum source 81 that maintains a flow 78 through the
discharge channel. As is also schematically indicated in FIG. 2,
the discharge channel 76 extends through a water collector 83 in
which the water removed from the layer 3 is separated from the
airflow and may be collected or discharged.
The airflow and can be reduced by reducing the height of the
interspace between the layer 3 and a boundary surface 83 facing the
layer into which the airflow is introduced. In turn, this is
advantageous for reducing disturbances caused by the two-phase flow
system. However, a flying height smaller than 2 .mu.m requires very
tight tolerances. Therefore, the airflow 71 is entered into an
interspace between the layer 3 and a boundary surface 83 having a
width (measured perpendicular to the layer 3) of at least 2 .mu.m
and preferably at least 5 .mu.m and at most 100 .mu.m and
preferably about 30 .mu.m.
The water 91 forms a film on the layer 3 having a thickness, and
wherein an interspace 86 between the layer 3 and a surface 87
facing the layer 3 upstream of the discharge channel 76, where the
water is discharged, is larger than the thickness of the film. This
facilitates the flow of water towards the discharge channel 76 and
reduced the pressure at which air needs to be supplied to reliably
ensure that no water passes beyond the zone 74.
The displacement of water at a relatively moderate flow rate of the
air supply is also enhanced by drawing away water and air from the
layer 3 at a higher flow rate than the sum of the flow rates at
which air and water are supplied via the channels 67 and 70. Thus,
all or virtually all air that is supplied via the channel 70 is
drawn towards water and a relatively moderate air supply pressure
is sufficient to ensure that all water is lead to the discharge
channel 76. The discharge channel 76 has a capacity larger than the
water and net upstream gas flow to avoid chaotic splashing at the
air-water interface. In the direction perpendicular of the movement
of the substrate and in the plane of the substrate, the size of the
slit and discharge channel 76 are preferably at least the size of
the liquid film.
Putting the gas supply hole close to the vacuum channel can reduce
the required gas pressure. Additional advantage of this system is
that most non-sticking particles will be removed too.
A constant and low flying height may be achieved by mounting the
proposed mechanism to an air bearing. Provided that the layer 3 is
sufficiently flat, this allows maintaining a constant and low
height without damaging the surface of the layer 3. The proposed
mechanism may even be integrated within the air bearing, utilizing
the high-pressure channel of the air bearing to push of the water
off the surface of the layer 3.
The air supply channel 70 and the water discharge channel 76 extend
through a structure that is rigidly connected to the lens system 9,
automatically ensuring that the flying height is kept constant. To
avoid the transfer of mechanical vibrations in the water removal
system to the lens system 9, it may, however, be advantageous to
only have a weak connection or axial guidance between the structure
containing the air supply channel 70 and the water discharge
channel 76, in particular if the supplied air ensures that the
distance between the surfaces 83 facing the layer 3 and the layer 3
are maintained within a range suitable for ensuring complete water
displacement and avoiding contact with the surface of the layer
3.
* * * * *